The present invention relates to the field of electroplating. More particularly, the present invention provides ultrasonically-enhanced electroplating apparatus and methods.
Plating a deep hole, channel, or other high aspect ratio structures, can pose challenging problems. During the plating of high aspect ratio structures, both mass transfer and electrochemical processes may be unfavorable, particularly at the deepest points in the structures. For example, it may be difficult for the bubbles generated during plating to be released from the high aspect ratio structures; metal ions may be depleted rapidly inside the structure and not be replenished properly; and undesirable decomposition products may not be easy to remove from the vicinity of the cathode. Further, the plating process tends to deposit thicker on the mouths of holes or the upper edges of channels, which can have a more significant impact on high aspect ratio plating. All these factors can introduce defects into the plating process.
Various methods have been developed for the high aspect ratio plating. In some LIGA (Lithographie, Galvanoformung and Abformung) processes, high aspect ratio structures have been plated with conventional plating process but with slower plating rate. The plating is usually on relatively small substrates, such as wafers. It is well recognized that traditional plating is both troublesome and time consuming. Plating of a high aspect ratio structure was conducted with a special designed instrument (Ariel G. Schrodt and Nick N. Issaev, xe2x80x9cEnhanced Microelectroforming Technology and Development of an Automated Microelectroforming Workstation,xe2x80x9d HARMST ""97 Worldwide LIGA Forum, June, 1997, Madison, Wis., Book of Abstracts). In this method, the part is plated while applying vacuum and thermal gradients. The plating can reach a high speed but can only plate small format with expensive instrument. Another approach to plating high aspect ratio structures involves pulse plating for filling recess of not more than about 1 micron in depth and width (U.S. Pat. No. 5,705,230).
Ultrasonic energy has been used in plating processes, most often as a cleaning aid. U.S. Pat. No. 5,705,230 does, however, use ultrasonic energy while plating a shallow recess. U.S. Pat. No. 4,842,699 describes using ultrasonic energy during via-hole plating to ensure sufficient electrolyte transport in the via-hole. U.S. Pat. No. 5,695,621 discloses the use of a resonating electroplating anode when plating inner surfaces of steam generator tubing. GB 2 313 605 discloses a chromium plating process employing ultrasonic energy to encourage release of bubbles. JP 1 294 888 A describes placing an ultrasonic vibrator inside of a cup for promoting gas bubble release. JP 51138538 discloses plating of a printed circuit board while using ultrasonic energy.
Although ultrasonic energy can enhance the mass transfer and removal of gas bubbles during plating, it can also have negative impacts on plating. In an electroforming process, inappropriate exposure to ultrasonic energy during electroforming can increase the residual stresses in the electroformed parts. The use of ultrasonic energy during electroplating can also cause adhesion problems between the deposited material and substrate, especially when a polymer or other non-conductive substrate is used.
Most electroplating processes are performed in plating tanks containing an electroplating bath. Another problem with the use of ultrasonic energy in a plating tank is that the energy distribution within the tank, especially on the cathode, is not uniform. The ultrasonic transducers are mounted in fixed locations on the side or bottom of the plating tanks, resulting with uneven ultrasonic energy distribution over the cathode because the ultrasonic energy is attenuated with distance. This problem becomes more acute when plating large surfaces because of the increased variations in energy distribution over the surface of the larger parts.
The present invention provides electroplating methods and systems employing ultrasonic energy to enhance electroplating processes. The electroplating methods involve locating an ultrasonic energy source between the anode and the cathode and sweeping a plating surface with ultrasonic energy having an area of maximum ultrasonic energy density. As a result, each portion of the plating surface receives varying amounts of ultrasonic energy during electroplating, with the maximum ultrasonic energy density being received intermittently by the plating surface.
The apparatus and methods of the present invention may provide particular advantages where the plating surface includes one or move cavities in which electroplating is desired. If the cavities, either holes formed through the cathode or wells formed in a surface of the cathode, have a relatively high aspect ratio, it may be difficult to electroplate the surfaces within the cavities. In some situations, the propagation axis of the ultrasonic energy (i.e., the direction of travel of the ultrasonic energy) may be aligned with the cavities such that the ultrasonic energy reaches throughout the cavities, thereby enhancing plating in the innermost portions of the cavities.
Another potential advantage of the methods and systems of the present invention is a reduction in the amount of ultrasonic energy needed to enhance electroplating. The amount of ultrasonic energy may be reduced because each part of the plating surface is intermittently exposed to the maximum ultrasonic energy density as the ultrasonic energy is swept across the plating surface.
Still another advantage of the present invention is that sweeping of the ultrasonic energy across the plating surface may reduce the problems associated with the use of ultrasonic energy during plating as discussed in the background, e.g., residual stresses, adhesion problems, etc. In addition, the sweeping nature of the ultrasonic energy may improve uniformity in the plated material.
A further advantage of the methods and systems of the present invention is that the ultrasonic energy impinges directly on the plating surface while movement of the ultrasonic energy source reduces or prevents problems associated with shielding or masking that can be caused by locating structures between the anode and the cathode. In those systems in which an ultrasonic energy source is moved between the anode and cathode during electroplating, the intermittent shielding of the cathode by the moving ultrasonic energy source may provide electroplating advantages similar to pulse plating processes (where the current density is intentionally varied).
Although the present invention may provide particular advantages when used in electroforming on high aspect ratio cavities, it may also be advantageous when used in connection with electroplating on any surface, whether or not that surface includes high aspect ratio cavities. Unless explicitly stated otherwise, the present invention is not to be limited to methods and/or systems for electroforming on high aspect ratio cavities.
In one aspect, the present invention provides an electroplating method that includes providing a tank containing a plating solution; providing an anode and a cathode within the plating solution, wherein the cathode has a plating surface; locating an ultrasonic energy source directly between the anode and the plating surface of the cathode; plating the plating surface of the cathode; and sweeping the plating surface with ultrasonic energy emitted by the ultrasonic energy source during the plating, wherein the sweeping includes moving an area of maximum ultrasonic energy density across the plating surface.
In another aspect, the present invention provides a method electroplating that includes providing a tank containing a plating solution; providing an anode and a cathode within the plating solution, wherein the cathode has a plating surface that includes a plurality of cavities, wherein each cavity of the plurality of cavities has a central axis and an aspect ratio of at least about 1:1 or higher; plating the plating surface of the cathode; locating an ultrasonic energy source directly between the anode and the plating surface of the cathode, wherein ultrasonic energy emitted by the ultrasonic energy source has a propagation axis; and sweeping the plating surface with ultrasonic energy emitted from the ultrasonic energy source during the plating.
The sweeping includes moving an area of maximum ultrasonic energy density across the plating surface with an area of maximum ultrasonic energy density; moving the plating surface and the ultrasonic energy source relative to each other while emitting ultrasonic energy from the ultrasonic energy source; and aligning the propagation axis of the ultrasonic energy with the central axis of each cavity of the plurality of cavities.
In another aspect, the present invention provides an electroplating apparatus with a tank having a tank volume; an anode located within the tank volume; a cathode located within the tank volume, wherein the cathode includes a plating surface; an ultrasonic energy source located within the tank volume, the ultrasonic energy source located directly between the anode and the cathode and oriented to emit ultrasonic energy at the plating surface; and movement apparatus providing relative movement between the ultrasonic energy source and the cathode while the ultrasonic energy source and the cathode are located within the tank volume.
These and other features and advantages of the present invention may be described below in connection with various illustrative embodiments of the methods and systems of the present invention.